docs: def print_topics(self, Nwords=10):
          """
          Print the top ``Nwords`` words for each topic.
          """
          print('Topic\tTop %i words' % Nwords)
          for k, words in self.list_topics(Nwords):
              print(unicode(k).ljust(3) + '\t' + ' '.join(list(zip(*words))[0]))